Metallocenes for the polymerization of olefins have been known for several years. European Patent Application EP 0416815 discloses constrained geometry metal complexes comprising a single cyclopentadienyl group and at least an heteroatom Y chosen from --O--, --S--, --NR--, --PR--.
These metallocenes have been synthesized by contacting a tetravalent metal reactant and a group 1 derivative or Grignard derivative of the cyclopentadienyl compound.
Methods for the preparation of metallocenes for the polymerization of olefins generally involve some variant of the reaction of a substituted cyclopentadienylalkali-metal salt with a transition metal halide in the proper molar ratio to produce the desired number of ligands to the transition metal. For instance, the U.S. Pat. No. 5,158,920 describes the preparation of isopropylidene (cyclopentadienyl-9-fluorenyl) zirconium dimethyl by deprotonation of fluorene (using methyllithium as deprotonating agent) to obtain the ionic species fluorenyl lithium, followed first by the ligand forming reaction with 6,6 dimethylfulvene, and then by the reaction with zirconium tetrachloride to form the metallocene.
These known methods are carried out under particular conditions incompatible with usual polymerization conditions. Consequently, the known methods,cannot be carried out "in situ" in the polymerization reactor but have to be carried out separately. In most cases the metallocenes obtained by these known methods have to be separated from the reaction mixture and stored under special conditions. Additionally, the known methods do not present a high productivity and a high selectivity.